Many different types of plastic bag making machines are known in the art of producing plastic bags for industrial and individual consumers for many different applications (e.g. small sandwich bags and trash bags). While the present invention has a wide range of applications for the production of such products, the related art will be explained by reference to one particular class of bags i.e., polyethylene trash bags or, garbage bags and wastebasket liners of the type usually sold in boxes of folded bags or rolls of bags.
Further discussion of the history and operation of these machines can be found in U.S. Pat. No. 4,642,084 (the '084 patent) entitled "Plastic Bag Making Machine", issued to Peter J. Gietman, Jr., on Feb. 10, 1987, and assigned to Custom Machinery Design, Inc. The '084 patent discloses a bag machine which includes a rotary drum with seal bars attached thereto and which includes a gear mechanism adapted for analog variation of the drum diameter between a first smaller diameter and a second larger diameter. Manual rotation of a hex nut assembly while the machine is stopped increases or decreases the drum's diameter through a series of appropriately mounted mitre gears and ring gears. Once this adjustment is made, the machine begins operation. Readjustment of the drum diameter can only be accomplished by stopping the machine to adjust the hex nut assembly.
Additionally, very small errors in drum diameter size can lead to acute problems, particularly an error in the seal to print registration distance that accumulates every revolution of the drum. An error of a fraction of an inch leads to serious problems when the bag width is only several inches across and the speed of the film moving through the machine is 500-900 bags per minute. By the time the error is detected, a considerable amount of film (or web) is wasted.
The control of the spacial relationship between a repetitive print pattern on the web and the repetitive seals the machine is placing across the web is referred to as the "registration" of the seal to the print on the web. This spacial relationship may also be referred to as the "phase" between the repetitive print and seal occurrences on the web.
Similarly, the control of the spatial relationship between the repetitive seals placed across the web and the repetitive perforations the machine is placing across the web is referred to as the "registration" of the perforation to the seal on the web. This spacial relationship may also be referred to as the "phase" between the repetitive perforations and the repetitive seals across the web. The distance between a seal and a perforation is commonly called the "skirt length" of the finished bag.
Another prior art device described in U.S. Pat. No. 4,934,993 (the '993 patent), also issued to Peter J. Gietman, Jr. and assigned to the assignee of the present invention, allows for adjusting the drum diameter while the bag making machine is in operation. The '993 patent requires the operator to preset the drum diameter corresponding to the nominal bag length, but will correct for slight variations in the bag length. If the seal is not properly registered to the printing on the bag the diameter of the drum is temporarily increased or decreased. When the registration is correct the drum returns to the preset diameter. One disadvantage of this system is that "hunting" (the drum diameter will continually change) will occur if the average bag varies more than slightly from the preset length. Hunting will be particularly prevalent at higher speeds.
When a bag making machine such as that described in the '993 patent is used to adjust the drum diameter, any device (such as a perforator, knife, die cutter, punching station, or folding station) on the bag making machine that processes the plastic downstream of the drum may become out of proper synchronization with the sealing process occurring in the drum while the drum is changing diameter. For example a perforator will be slightly out of synchronization causing perforation to seal registration (skirt length) to vary. According to the '993 patent the skirt length may be adjusted manually. However, by the time the error is detected and the manual correction made, a considerable amount of film may be wasted.
Another device, disclosed in U.S. Pat. No. 5,292,299 (the '299 patent), uses a proximity detector and an encoder to determine where each seal will be placed. However, the '299 patent does not actually sense and determine the location of a seal; the '299 patent "fixes" the distance between the point of application of the seal and the point of perforation at a constant minimum distance instead of detecting it. Because the actual location of the seal is not known, the user is required manually to initialize the skirt length, which requires time to accomplish and may result in errors and undue waste. Likewise, if synchronization is lost, errors will occur in the placement of the perforation.
In any event, the prior art and the '299 patent demonstrate the desirability of determining the location where the perforation should be placed, with respect to the seal.
As one alternative, this may be done by detecting a printed mark, and locating both the seal and perforation with respect to the printed mark. One known arrangement for detecting printed marks, such as registration marks, on film (or web) materials involves the use of a light source aligned generally perpendicular to the web and a sensor positioned substantially collinear with the light source.
While this prior art method indirectly locates the perforation with respect to the seal, it would be advantageous to directly place the perforation with respect to the seal. The distance between the location where the seal is created, and where the perforation is created can be relatively large (14 feet or more in some designs using a downstream folding board). This large distance increases the likelihood of errors when linking the location of the perforation to the print or the location where the seal was created. Additional error may occur when a downstream folding station (between the seal drum and the perforator) is used, because the bags are folded under tension, but sealed when the film is relaxed: thus, the film may stretch after being sealed. Also, shrinkage can occur as a result of the heat seal. Thus, it is desirable detect a seal close to the perforating station.
However, known print detectors are not successful at detecting features in certain sheet materials, and in plastic film materials in particular. Accordingly, it would be desirable to provide the capability to detect a seal in a plastic film and control film perforation such that perforations are directly registered with respect to the seal. Additionally, it may be desirable to detect a perforation for properly separating bags being removed from a roll of bags and/or folded. Thus, it would be desirable to provide a seal detection method and arrangement which can accurately and consistently detect a seal in a moving film, even where the film is moving at linear speeds in excess of 600 feet per minute.
A bag making machine which overcomes the foregoing shortcomings and satisfies these needs would represent a considerable advancement in the art.